1) Field of the Invention
The present invention is directed to a method and system for estimating local towed array angles in order to improve acoustic beamforming during turns.
2) Description of Prior Art
Direction-finding devices are used to determine the direction of propagating signals and hence the location of the source of these signals. These devices are utilized in spectrum monitoring, reconnaissance and surveillance. Direction finders utilize arrangements or arrays of receivers to detect arriving signals and to determine the direction from which these signals originated. For example, appropriate time delays applied to the receivers serves to steer the array and to calculate the direction of arrival of the impinging signals. Therefore, the data gathered from the receivers are used in conjunction with data processing algorithms to interpret the data and determine signal propagation direction relative to the receivers.
When processing data from an array of acoustic sensors, accurate knowledge of the shape and location of the array is indispensable. For example, towed arrays suffer degraded detection and tracking capability during turns, partly due to imperfect knowledge of their shape, leading to erroneous beamformer shape correction. Software for predicting the shape of towed arrays as a function of time has been developed. The accuracy of these computational models is limited due to uncertainty in the drag coefficients for cases where the angle of tow is small-typically from zero to ten degrees. The ability to determine the local angles of an array at discrete locations along the array in real time is therefore highly advantageous.
For small angles, the turbulent boundary layer on the array undergoes structural changes, and the mean and fluctuating wall shear stress field at the fluid/solid interface changes. These changes are reflected in the space-time correlation coefficient of the wall shear stress field. A direct measurement of the space-time correlation function with pairs of wall shear stress sensors along the array therefore provides sufficient information to estimate the local angle of attack.
Currently, towed array heading sensors and depth sensors are used to estimate array shapes. The pms (position measurement system) for determining the relative positions of multi-line arrays is not suitable for use in a single array where small angles are involved. Pitot tubes or velocity probes are not practical due to mechanical constraints and incompatibility with the handling systems. Also, GPS sensors are not feasible due to the underwater application and operational depths. Lastly, real time computational models suffer from inaccuracy due to uncertainty in drag coefficients.